scripts/glide-update: ensure running from $GOPATH
scripts/glide-update: ensure running from $GOPATH

glide is confused when it's not running the rkt repository inside $GOPATH.
The new check implies to be in the root directory of the repo.

ybubnov/rkt,achanda/rkt,joshix/rkt,yifan-gu/rocket,Wilybald/rkt,joshix/rkt,sgotti/rkt,s-urbaniak/rkt,dgonyeo/rkt,intelsdi-x/rkt,yifan-gu/rkt,shobhit85/rkt,dgonyeo/rkt,lucasallan/rkt,yifan-gu/rkt,coreos/rkt,coreos/rkt,monstermunchkin/rkt,akshaykarle/rkt,achanda/rkt,squeed/rkt,euank/rkt,sstabellini/rkt,yifan-gu/rkt,s-urbaniak/rkt,ybubnov/rkt,lucasallan/rkt,jakon89/rkt,jjlakis/rkt,jonboulle/rkt,joshix/rkt,nhlfr/rkt,sgotti/rkt,s-urbaniak/rkt,dgonyeo/rkt,kinvolk/rkt,cab105/rkt,nhlfr/rkt,cab105/rkt,jonboulle/rkt,kinvolk/rkt,jjlakis/rkt,Wilybald/rkt,squeed/rkt,jakon89/rkt,lucasallan/rkt,ybubnov/rkt,nhlfr/rkt,rkt/rkt,sstabellini/rkt,cab105/rkt,rkt/rkt,joshix/rkt,yifan-gu/rocket,monstermunchkin/rkt,alban/rkt,euank/rkt,jonboulle/rkt,Shashwatsh/rkt,coreos/rkt,Wilybald/rkt,rkt/rkt,shobhit85/rkt,kinvolk/rkt,intelsdi-x/rkt,alban/rkt,alban/rkt,Shashwatsh/rkt,lucasallan/rkt,intelsdi-x/rkt,rkt/rkt,euank/rkt,Wilybald/rkt,Shashwatsh/rkt,sgotti/rkt,kinvolk/rkt,shobhit85/rkt,akshaykarle/rkt,squeed/rkt,sstabellini/rkt,yifan-gu/rocket,jakon89/rkt,achanda/rkt,jjlakis/rkt,monstermunchkin/rkt,akshaykarle/rkt